The invention relates to a photomask comprising a base plate of a radiation-transmitting and electrically insulating material, a first side of which is provided with a layer of an electroconductive mask material wherein a mask pattern to be imaged is formed which is enclosed by an inner edge and an outer edge of a mask material, which edges are electrically separated from each other by a ring-shaped protection area.
Such a photomask can particularly suitably be used in the manufacture of semiconductor devices and flat panel displays. In practice, the base plate generally is a plate of quartz glass, the layer of mask material generally contains chromium. The pattern of the mask is imaged, often on a reduced scale, on a layer of a photoresist which is provided, for example, on a layer of a metal. After exposure and development of the photoresist, the layer of metal can be etched in a pattern of conductor tracks corresponding to the mask pattern. The photomask pattern to be imaged comprises many closely spaced tracks of a mask material which are sometimes connected to larger areas of a mask material. The tracks correspond to the conductor tracks to be formed in the layer of metal, and the areas correspond to, for example, bond pads to be formed in the metal layer, which bond pads are used for contacting the devices to be manufactured.
The photomask has an edge of a conductive mask material, which is divided into an inner edge and an outer edge which are electrically separated from each other by a ring-shaped protection area. This ring-shaped area serves to protect the photomask against damage by electrostatic discharges (Electro-Static Discharge) which may occur in the mask pattern.
In operation, a photomask having a mask pattern which is enclosed by a closed edge of a conductive mask material, damage due to electrostatic discharges may occur. The mask material present on the electrically insulating base plate may become electrically charged. This may be caused, for example, by air flowing past or by friction caused by contact with clothing and other insulating materials. In order to be able to image the photomask onto the layer of photoresist, the photomask is placed in a projector on a grounded mask holder. In this manner, the closed edge of masking material present around the mask pattern is grounded. Between this edge and the mask pattern voltage differences may develop whose magnitude is such that electric discharges occur in the mask pattern leading to said damage.
In JP-A-5-100410, a description is given of a photomask of the type mentioned in the opening paragraph, in which photomask the mask material is entirely removed from the ring-shaped protection area. This empty ring-shaped area has a width ranging from 0.5 to 2 mm.
If the conductive mask material is electrically charged as described hereinabove, then only the outer edge is grounded if the photomask is placed in a projector. If the empty protection area sufficiently insulates the outer edge and the inner edge from each other, then no voltage difference between the inner edge and the mask pattern will develop. Electrostatic discharges which may damage the mask pattern seem to be precluded in this way. However, it has been found in practice that charge present on the inner edge and in the mask pattern can cause damage in spite of the presence of the empty ring-shaped protection area.